From 63437ffbb58837b214b4b92cb1c54bc5f3279928 Mon Sep 17 00:00:00 2001
From: Simon Kelley <simon@thekelleys.org.uk>
Date: Wed, 6 Sep 2017 22:34:21 +0100
Subject: [PATCH] Fix CVE-2017-13704, which resulted in a crash on a large DNS
 query.

A DNS query recieved by UDP which exceeds 512 bytes (or the EDNS0 packet size,
if different.) is enough to cause SIGSEGV.
---
 CHANGELOG     |    7 +++++++
 src/auth.c    |    5 -----
 src/forward.c |    8 ++++++++
 src/rfc1035.c |    5 -----
 4 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/CHANGELOG b/CHANGELOG
index 3a640f3..7e65912 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -17,6 +17,13 @@ version 2.78
 	--strict-order active. Thanks to Hans Dedecker
 	for the patch
 
+	Fix regression in 2.77, ironically added as a security
+	improvement, which resulted in a crash when a DNS
+	query exceeded 512 bytes (or the EDNS0 packet size,
+	if different.) Thanks to Christian Kujau, Arne Woerner
+	Juan Manuel Fernandez and Kevin Darbyshire-Bryant for
+	chasing this one down.  CVE-2017-13704 applies.
+
 	
 version 2.77
 	Generate an error when configured with a CNAME loop,
diff --git a/src/auth.c b/src/auth.c
index 2c24e16..7f95f98 100644
--- a/src/auth.c
+++ b/src/auth.c
@@ -119,11 +119,6 @@ size_t answer_auth(struct dns_header *header, char *limit, size_t qlen, time_t n
   struct cname *a, *candidate;
   unsigned int wclen;
   
-  /* Clear buffer beyond request to avoid risk of
-     information disclosure. */
-  memset(((char *)header) + qlen, 0, 
-	 (limit - ((char *)header)) - qlen);
-  
   if (ntohs(header->qdcount) == 0 || OPCODE(header) != QUERY )
     return 0;
 
diff --git a/src/forward.c b/src/forward.c
index f22556a..e3fa94b 100644
--- a/src/forward.c
+++ b/src/forward.c
@@ -1188,6 +1188,10 @@ void receive_query(struct listener *listen, time_t now)
       (msg.msg_flags & MSG_TRUNC) ||
       (header->hb3 & HB3_QR))
     return;
+
+  /* Clear buffer beyond request to avoid risk of
+     information disclosure. */
+  memset(daemon->packet + n, 0, daemon->edns_pktsz - n);
   
   source_addr.sa.sa_family = listen->family;
   
@@ -1688,6 +1692,10 @@ unsigned char *tcp_request(int confd, time_t now,
   
       if (size < (int)sizeof(struct dns_header))
 	continue;
+
+      /* Clear buffer beyond request to avoid risk of
+	 information disclosure. */
+      memset(payload + size, 0, 65536 - size);
       
       query_count++;
 
diff --git a/src/rfc1035.c b/src/rfc1035.c
index 26f5301..af2fe46 100644
--- a/src/rfc1035.c
+++ b/src/rfc1035.c
@@ -1223,11 +1223,6 @@ size_t answer_request(struct dns_header *header, char *limit, size_t qlen,
   struct mx_srv_record *rec;
   size_t len;
 
-  /* Clear buffer beyond request to avoid risk of
-     information disclosure. */
-  memset(((char *)header) + qlen, 0, 
-	 (limit - ((char *)header)) - qlen);
-  
   if (ntohs(header->ancount) != 0 ||
       ntohs(header->nscount) != 0 ||
       ntohs(header->qdcount) == 0 || 
-- 
1.7.10.4